1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus. In particular, the present invention relates to an imaging lens having a wide angle of view and a long back focus, and an imaging apparatus including the imaging lens.
2. Description of the Related Art
Conventionally, small wide-angle imaging lenses were used in the fields of in-vehicle cameras, surveillance cameras, endoscopes and the like. In recent years, an imaging lens is generally used in combination with an imaging device that images an image formed by the imaging lens. In such a case, an optical low-pass filter, an infrared ray cut filter and the like are often placed between the lens system and the imaging device. Therefore, a sufficient back focus is needed. Further, in some type of endoscope, an imaging plane of an imaging device is arranged parallel to the long axial direction of an insertion unit of the endoscope. In such a type of endoscope, an optical path conversion member, such as an optical path conversion prism, is generally inserted between the imaging lens and the imaging device to change the direction of the optical path. Therefore, a sufficiently long back focus is needed. Meanwhile, it is necessary to make a focal length short to obtain a wide angle of view, and it is difficult to obtain a long back focus and a wide angle of view at the same time. U.S. Patent Application Publication No. 20040125469 (Patent Document 1), Japanese Unexamined Patent Publication No. 63(1988)-261213 (Patent Document 2), Japanese Unexamined Patent Publication No. 2008-257108 (Patent Document 3), Japanese Unexamined Patent Publication No. 2008-257109 (Patent Document 4), and U.S. Patent Application Publication No. 20100305405 (Patent Document 5) disclose imaging lenses for endoscopes invented by the inventor of the present invention, and in which both a long back focus and a wide angle of view are realized.